Dynamic reliability based design optimization of the tripod sub-structure of offshore wind turbines

This work presents an efficient methodology for the Reliability Based Design Optimization (RBDO) of the tripod sub-structure of offshore wind turbines considering dynamic response requirements. The cost of supporting structure of offshore wind turbines is so high that optimization in the design stage is a basic requirement. Traditional design optimization methodology for offshore structures uses deterministic modeling. However, the existence of uncertainties, such as manufacturing tolerances, material properties, and environmental loads, requires a probabilistic optimization technique. Uncertainties in the offshore wind turbines design process may have a strong effect on its dynamic responses but very little researches have been conducted to incorporate the uncertainty property into design optimization of the supporting structures. In this study, a framework of a dynamic reliability based design optimization for tripod sub-structures was proposed. Firstly, a Finite Element (FE) model of a tripod sub-structure of the NERL 5 MW wind turbine was constructed for dynamic response analysis in time domain and generating several accurate sampling points. Secondly, an efficient approximate model was built utilizing these sampling points to replace the original time consuming dynamic response analysis of FE model. At last, this approximate model was used during the optimum iterative procedure with a global optimization algorithm to gain the final best design point considering uncertainties. In this optimization methodology, some sizes of structural components, applied loads, and some material properties are considered as random variables. The structural stress and natural frequency are considered as constraints and the weight of the structure is considered as the objective function. The reliability of the structure is finally determined through Monte Carlo simulations. The results show that the proposed methodology can obtain a reliable design with better dynamic performance and less weight. Compared with the deterministic optimization, the presented dynamic Reliability Based Design Optimization of tripod sub-structure of offshore wind turbines is more rational and practical and this efficient methodology can be applied in the design of other similar offshore structures.